A handful of elements that make up the iOS UI in its entirety are round or have rounded corners; this includes, but is not limited to: passcode buttons, Control Center toggles, the corners of banner notifications, and Home screen folders, among other things.
If you're not a fan of this system-wide rounded style, a new free jailbreak tweak called ringUI by CydiaGeek can be installed to make several elements on your device look much squarer.
After you install ringUI, a new preferences pane is added to the Settings app so you can remove the roundness effects from a number of things in iOS: